Ionizer emission and filament current regulating circuit

ABSTRACT

A control circuit for an ionizer used in a quadrupole mass analyzer. The circuit relates the current supplied to the filaments (filament current) and the current output of the ionizer (emission current) such that when a desired emission current level is established, the circuit provides a filament current necessary to produce that emission current level. The emission current is sensed and compared with a reference level for this current in a comparator. The comparator output appropriately braces a current-regulating transistor in the filament supply as a function of the comparison.

United States Patent {54} IONIZER EMISSION AND FILAMENT CURRENT [56] References Cited UNITED STATES PATENTS 3,441,833 4/l969 Bahrs et al. 323/9 Primary ExaminerGerald Goldberg Att0rneys-Edward A. Petko and Robert M. Skolnik ABSTRACT: A control circuit for an ionizer used in a quadrupole mass analyzer. The circuit relates the current supplied to Egp i g Q the filaments (filament current) and the current output of the ionizer (emission current) such that when a desired emission [S2] LS. Cl 323/4, current level is established, the circuit provides a filament cur- 323/9,323/22 T, rent necessary to produce that emission current level. The [51] lnLCl G05t' 1/58, emission current is sensed and compared with a reference G05f 1/64 level for this current in a comparator. The comparator output [50] Field of Search 323/2, 4, 9, appropriately braces a current-regulating transistor in the fila- 22 T, l622 mcnt supply as a function of the comparison.

r1 cm 8 03 R2 at l C3 R6 02 RIO Q4 +1 l R9 7 R3 R4 OI Rll FILAMENT l CR3 8 CR2 R7 m2 T 1 CR4 47C fi 1 R20 CR9 C6 POWER SUPPLY FROM 4 CR8 FARADAY R2| CAGE 6 CRII 07 '2 care R22 R23 R24 i R26 R27 PATENTEI] AUG] 0 15m Em u wNm mmm mm NNm me 5m mu Jo :BEE 95 v 20% 0% w N ,INVENTOR. ROBERT M. BRYNDZA IONIZER EMISSION AND FILAMENT CURRENT REGULATING CIRCUIT This invention relates to a circuit for controlling the emission current in the ionizer section of a quadrupole mass analyzer.

The quadrupole mass analyzer has been described 91 the. prior'art, particularly in.U.S. Pat. No. 2,939,952 to Paul. The analyzer consists of three major components: the ionizer; the

quadrupole mass filter; and the ion detector. The substance to be analyzed is introduced into the ionizing volume where a number of ions are created by electron impact. These ions are accelerated and focused into thefilter section. The filter allows only those ions of a specific range of charge-to-mass ratios to pass through and reach the detector. The detector may be either an electron multiplier or a Faraday Cup.

The output current of the detector is a measure of the number of atoms or molecules in the analyzed substance which, when ionized, have a particular charge-to-mass ratio. The charge-to-mass ratio is determined by the voltages applied to the filter section. In addition, since the majority of the ions which are produced in the ionizer are singly charged, the mass of the atoms or molecules may be directly determined. A display of the quantitative abundance as a function of atomic mass can be conveniently presented on conventional readout devices such as oscilloscopes and recorders.

The ability of the quadrupole mass filter to separate ions of different charge-to-mass ratios is due to the mass filtering action of the quadrupole section. This mass filtering action is based on the behavior of the-equations of motion of charged particles injected into the quadrupole analyzing field. The quadrupole assembly includes four rods of stainless steel or molybdenum. These rods are accurately located in a rectangular array by means of an arrangement of insulators. The dimensions of the array are chosen such that a best approximation to a set of hyperbolic-shaped rods results. Superimposed RF and DC voltages with sawtooth waveforms are applied to electrically paired rods. One pair of rods is at a positive potential with the other pair at a negative potential. The potential 1 at any point in the electrostatic field thus created within the rod array can be represented by:

V =DC amplitude;

V =RF amplitude;

r =radius ofrectangular array;

x,y=perpendiculars to the axis of the filter section.

The electric field distribution in the region is given by the first derivative of q and the force on a singly charged ion of mass m subjected to the electric field has the form of the well known Mathieu equation:

The Mathieu equation is a linear second order differential equation with constant periodic coefficient. Investigation of the stability of simultaneous solution of the Mathieu equations have shown several stable regions. The largest one of these is chosen for quadrupole operation.

The ionizer includes a heated filament which emits a stream of electrons. The electrons bombard the atoms or molecules of the substance to be analyzed producing positive and negative ions and fragmented particles. The ions are then accelerated and focused into the filter section by electrostatic lens. Current flow through the filament (filament current) causes it to heat and, at some current value, electrons will be emitted.

The filament block supports the filament and aligns it with the electron slit. The filament block and filament operate at a fixed negative DC voltage level with respect to the Faraday cage. This fixed voltage imparts the required energy to the bombarding electrons to ionize the substance to be analyzed. One side of the'filament is electrically connected to the filament block.

The electron extractor attracts 0r collects electrons emitted by the filament and any secondary electrons which result from electron bombardment of the metal surface of the extractor. All electrons which reach the extractor or the Faraday cage from the filament, constitute the total emission current.

The Faraday cage provides:

a. molecular entrance aperture;

b. electron slit;

c. ion exit aperture.

Gaseous molecules enter the ionization chamber through the electron slit. The slit confines the shape of the electron beam which emerges therefrom to a fan-shaped wedge. The energy of the electrons in the beam is a function of the potential difference between the extractor and Faraday cage, and the filament block.

The Faraday cage operates at a fixed positive DC potential with respect to ground and it is this voltage which imparts initial energy to the positive ions formed and moves them out of the ionizing chamber through the ion exit aperture and toward the filter rods. The ionizing chamber is surrounded on three sides by the Faraday cage and on the fourth side by the electron extractor. The two ends are open.

Since the Faraday cage is at a positive potential, it will attract some of the electrons (emission current) emitted by the filament but the number is insufficient to be detrimental to the ionization process.

The ions then pass through the ion exit aperture with an energy, eV,, due to the positive potential on the aperture. The

focus lens mounts adjacent to the ion exit aperture and is electrically insulated from it. Its potential may be varied over the range 0 to -l00 volts. This negative potential further accelerates the positive ions and imparts to them additional energy. The present invention relates to an electronic current-regulating circuit to control the filament current and the emission current in the ionizer. The circuit consists of three main modules: the AC power supply; the filament current preregulator; and the emission current regulator. Application of AC voltage from the AC power supply activates the filament current regulator. The maximum amount of filament current can be adjusted by setting a bias potentiometer provided for that purpose. The potentiometer also provides an upper limit of safe" current which the filament can withstand. This filament current regulator isindependent of emission current and only insures that adequate filament current will be available for a desired emission current range. Thus, when the actual emission current attains the value desired, the circuit ensures that the necessary filament current is provided.

It is an object of the present invention to provide a simplified circuit for controlling the filament circuit supplied to the ionizer of a quadrupole mass analyzer.

Another object of the invention resides in the provision of 4 control circuit, which, by regulating emission current ensures that the filament current supplied to the ionizer will be at the required value.

A further object of the present invention is a control circuit for an ionizer whereby the relationship between filament current and emission current is maintained at an optimum condition.

These as well as further objects and advantages ofthe invention will become apparent to those skilled in the art from a reading of the following specification in which reference is made to the following drawings: the single FlGURE is a schematic diagram of the preferred embodiment of the emission current regulating circuit.

Turning to the drawing, a source of AC supply voltage is connected across terminals 1 and 2. This voltage is rectified by a diode bridge circuit CR1 coupled across terminals 1 and 2 via the primary and secondary windings of transformer T1. Transformer Tl reduces the magnitude of the AC supply. The rectified signal is filtered by a low-pass filter R1 and C1. The filtered signal thus generated is applied to a current regulator circuit including transistors Q1 through Q4 and their associated components.

More particularly, the juncture or resistor R1 and capacitor C1 is connected to the base of transistor Q1 via a bias adjusting network. In the network, resistor R2 and capacitor C2 set a DC voltage level for the base. Zener diode CR2 connected between the juncture of resistor R2 and capacitor C2 stabilizes this level. Fixed resistor R4 and adjustable potentiometer R3 permit variation of the bias applied to Q1.

Diode CR3 and resistor R7 form part of the bias circuit for transistor Q1: diode CR3 providing temperature compensation for the transistor. The output of the circuit at terminals 7 and 8 is taken from the emitters of transistors Q1 and Q4. Diode CR4 clamps the base of the transistor preventing it from going positive during operation. Capacitor C3 is connected between the collector and the base of transistor Q1 for frequency compensation.

The signal at the collector of transistor Q1 is connected to the base of transistor Q2 via load resistor R6. Bias for Q2 is set by resistor R5. Transistors Q2 and Q3 provide current amplification. Resistor R8 is the bias resistor for transistor Q3 while resistor R9 connected to the collector of transistor Q2 provides current limiting. The output of transistor Q3 is connected to a power amplification stage including transistor Q4. Bias for this stage is provided by resistors R10 and R11.

The current output of transistor Q4 is degeneratively fed back to the emitter of transistor Q1. More particularly, resistor R12 samples the current output of transistor Q4 which tends to stabilize the current supplied to terminals 7 and 8. The filament of the ionizer is connected across these terminals.

The present circuit provides filament current control based on the emission current desired. When actual emission current is equal to desired emission current, the circuit ensures that the filament current supplied will be sufficient to produce the desired emission current.

Toward this end, a power supply 3 having a regulated output of 100 volts is provided. The power supply voltage is applied across Zener diode CR8 which sets a reference level for the remaining circuit elements. When emission current flows, a voltage drop is produced across resistor R20. Emission current is detected at terminal 4. This voltage drop is compared by amplifier 6 with a reference set by a potentiometer, R31. As soon as the voltage drop produced across resistor R20 is equal to the voltage set by potentiometer R31, the transistor Q7 provides a control current to the base of transistor Q11 in the filament current regulator to reduce the filament current from its previous magnitude. More particularly, the input supplied to amplifier 6 from resistor R20 is fed from the Faraday cage of the ionizer and directly represents the emission current. Diodes CR9 and CRllt) bias amplifier 6 to its active region. Transistors Q8 and Q9 are constant current sources for the Zener diodes CR8, CR9 and CRUD. Resistors R22 and R23 provide bias for transistor Q8 while resistors R26 and R27 are the bias resistors for transistor Q9. Balance resistors R24 and R25 set the bias for the constant current source.

For transistor Q7, diode CR11 clamps the base to prevent it from going positive. Resistor R21 is the bias resistor for this transistor while capacitor C6 provides frequency compensation. Resistor R28 connected between the collector of transistor Q7 and the base of transistor O1 is the load resistor for Q7.

What I claim is:

1. A current-regulating circuit for producing a first current output as a function ofa second current input comprising;

a first potentiometer for producing a variable electrical signal;

a source of control current;

a differential amplifier having a first input connected to said potentiometer and a second input connected to said current source for producing an output signal proportional to the difference between said inputs;

a source of control voltage;

a first voltage amplifying transistor having a base element, a

collector element and an emitter element; a second potentlometer connected between said voltage source and said base element for limiting the voltage ap plied to said base;

means connected to said base element and to said differential amplifier for degeneratively applying said amplifier output to said base element, thereby reducing the output of said first transistor,

serially connected cascade first and second current-amplifying transistor connected to said collector element for amplifying the current produced by said first voltage amplifying transistor;

a second voltage amplifier transistor connected to said second current-amplifying transistor for providing further amplification and feedback means connecting said first and second voltage-amplifying transistors to stabilize the output of said second voltage-amplifying transistor, and output means connected to said last-mentioned transistor for supplying a regulated output current. 

1. A current-regulating circuit for producing a first current output as a function of a second current input comprising; a first potentiometer for producing a variable electrical signal; a source of control current; a differential amplifier having a first input connected to said potentiometer and a second input connected to said current source for producing an output signal proportional to the difference between said inputs; a source of control voltage; a first voltage amplifying transistor having a base element, a collector element and an emitter element; a second potentiometer connected between said voltage source and said base element for limiting the voltage applied to said base; means connected to said base element and to said differential amplifier for degeneratively applying said amplifier output to said base element, thereby reducing the output of said first transistor, serially connected cascade first and second current-amplifying transistors connected to said collector element for amplifying the current produced by said first voltage amplifying transistor; a second voltage amplifier transistor connected to said second current-amplifying transistor for providing further amplification and feedback means connecting said first and second voltage-amplifying transistors to stabilize the output of said second voltage-amplifying transistor, and output means connected to said last-mentioned transistor for supplying a regulated output current. 